1. Field of the Invention
The present invention relates to superlurinescent diodes (SLDs). In particular, it relates to wavelength stabilization of SLDs, optical amplifiers, and other broadband light sources by comparing the output at two or more wavelengths and using an error signal for closed loop feedback for temperature or current control.
2. Description of the Prior Art
An interferometric fiber optic gyroscope (IFOG) is an instrument that measures the phase shift or time delay between counter-propagating lights injected in an optical fiber loop on a rotating platform and converts it to a measure of rotation rate. The measured phase shift, called the "Sagnac" effect, is proportional to the rotation rate. The proportionality constant, called the "scale factor", is proportional to the average wavelength of the light source. Hence, the accuracy of the IFOG is determined by the accuracy by which the average wavelength of the light source is known. Thus, the design of an interferometric fiber optic sensing system depends on the stability of the wavelength of the optical source. In particular, for the fiber optic rotation sensors to be useful in navigation, the emission wavelength of the optical source must be controlled to ten parts in a million or better.
Wavelength stabilization of the optical source has been accomplished by using a thermally-compensated optical interferometer to detect changes in the emission wavelength of the source. In a thermally-compensated interferometer, the interferometer is designed so that shifts occur in the interferometer only in response to shifts in the wavelength of the incident light and not in response to changes in ambient temperature. Shifts in the source wavelength generate an error signal in the interferometer which is used to correct the emission wavelength of the source.
In order to avoid coherence noise effects, the light source is required to have a broad bandwidth. The preferred broadband light source is the super-luminescent diode (SLD). The SLD emits in a broad spectral band which reduces phase noise in the sensor caused by the Kerr effect and by coherent backscatter in the fiber. In the SLD, the optical feedback present in laser diodes is suppressed by placing the current stripe at an angle with respect to the diode facets so that the reflected light is away from the stripe. A typical angle is about 5.degree. to 10.degree.. Because of the absence of feedback, these devices emit only incoherent light, the type of light that is required by the fiber optic rotation sensor. However, the SLD optical spectrum varies with temperature at a rate of about 350 ppm/.degree. C. (parts per million per .degree. C.), and for navigational grade IFOGs, this variation should be reduced to below 10 ppm/.degree. C. The temperature of the SLD is typically held to within 0.1 to 0.2.degree. C. by means of a thermoelectric cooler (TEC), but this does not provide sufficient accuracy for navigation.
Although angled stripe SLDs are appropriate as sources for fiber optic rotation sensors, high accuracy control of the emission wavelength of these devices by interferometry is made difficult by the short coherence length (broad bandwidth) of the source. For these devices, this length is typically on the order of 20 .mu.m to 60 .mu.m. To generate an error signal in response to changes in the emission wavelength of the source, interferometers must therefore be designed and fabricated with optical pathlength differences on the order of the coherence length of the SLD. Such interferometers are difficult and expensive to fabricate.
What is needed is a simple means to measure the wavelength very accurately by taking advantage of the broad bandwith, and to use the information in a closed loop feedback system to continually adjust the current supplied to the drivers to maintain that value of the wavelength. The present invention has been developed for this purpose.